Two-telephone intercoms are well known. The two telephones are often connected in series with a dc power source, i.e., a battery. Such intercom circuits sometimes include buzzer arrangements for ringing the opposite telephone; and these buzzer arrangements usually include at least one "ringing" switch, which must be operated by the calling party, the buzzer of the second telephone being operated only as long as the buzzer switch at the first unit is held in its closed position.
This invention greatly simplifies the signalling circuit used to call the second telephone.